From semiconductor lasers to fiber Bragg grating lasers in optical communications

Citation

Provenzano, Dan Raymond (2001) From semiconductor lasers to fiber Bragg grating lasers in optical communications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/setx-7j50. https://resolver.caltech.edu/CaltechETD:etd-06132005-083934

Abstract

This thesis describes the semiconductor laser signal and noise propagation effects in single mode fiber and fiber Bragg gratings. The capability to fabricate custom fiber Bragg gratings was developed, which enabled the design and fabrication of gratings for a number of applications. Finally, gratings were developed and specialized for use in a single mode fiber ring laser. A quantum mechanical description of laser noise is presented in order to discuss pump-noise suppressed semiconductor lasers capable of sub-shot noise, also known as "squeezing." Experimental results for an 850 nm Fabry-Perot semiconductor laser are presented showing squeezing at room temperature of 29% below the shot noise limit measured using a balanced homodyne detector, corresponding to 41% below the standard quantum limit at the output facet of the laser. The side mode suppression ratio was varied with slight temperature tunings and correlated with the laser noise. It was found that the higher the sidemode suppression ratio, the lower the noise. Noise analysis was continued with 1540 nm distributed feedback semiconductor lasers. Laser parameters such as noise, chirp, and resonance frequency were characterized by propagation in dispersive fiber,and fitting the parameters to a model for the fiber. Again, a correlation was found between side mode suppression and laser noise, especially after several kilometers of propagation in fiber. The principles of signal and noise propagation were applied to fiber Bragg gratings. Theory and experiment indicated direct laser modulation enhancement by a uniform fiber Bragg grating by 7 dB at modulation frequencies, of up to 25 GHz, and also noise reduction of 2 dB at frequencies up to 15 GHz. Facilities were established to write and produce customized fiber Bragg gratings of various strengths in various fiber types as well as in ion-exchanged waveguides in bulk glasses. Analyses of writing times and strengths were performed and optimized for various applications. Uses for these gratings include dispersion compensation, noise reduction, beam or pulse shaping, and spectral filtering for dense wavelength division multiplexed (DWDM) optical networks. Amplitude and phase masks were developed and shown to produce arbitrarily apodized and chirped gratings. Fiber gratings were next refined for use as key elements in a new type of single mode fiber ring laser. Some of the beneficial characteristics of this fiber laser include long cavity size (80 cm), 80 dB signal-to-noise ratio, high side mode suppression ratio, and white noise linewidth as narrow as 2 kHz. The laser noise was also nearly shot noise limited. This combination of low amplitude and low phase noise allowed the observation of extremely low noise enhancement after 50 km of standard, dispersive fiber up to 20 GHz frequency. A comparison was made between our fiber ring laser and a standard high grade distributed feedback semiconductor laser in transmitting 10 Gbits/sec data. Over a 50 km fiber, the fiber ring laser achieved the same signal to noise ratio with half the power as the semiconductor laser.

Thesis Files